Superabsorbent polymers (SAPs) have wide applications. For example, they are used in a variety of medical and hygiene products.
These polymers, as hydrophilic crosslinked networks, have high water absorbing capacity. In deionized water, current SAP networks can absorb 900 times its weight, i.e., a water absorbency of 900 g/g. However, they are not very effective in salt-containing water. In a 0.9% saline solution, their water absorbency drops significantly to about 70 g/g. See Jim Sheu, U.S. Pat. No. 5,163,115. In a pressurized saline solution, their water absorbency decreases further to about 35 g/g.
Itaconic acid is widely used as a monomer to prepare SAP networks. In general, it is co-polymerized with another monomer, e.g., acrylic acid and acrylamide. The copolymers thus formed are then crosslinked to form a SAP network. See Rodriguez and Katime, Macromolecular Materials and Engineering, 288, 607-12 (2003). Itaconic acid has two polar carboxyl groups, which attribute to the hydrophilicity and thus water absorbency of the SAP network. Chain transfer reactions are common in itaconic acid copolymerization, resulting in low-molecular-weight SAP networks. Such networks, which have low crosslinking density and low mechanical strength, absorb water poorly in a pressurized saline solution. See Rodriguez and Katime (2003); and Pulat and Eksi, Journal of Applied Polymer Science, 102, 5994-99 (2006).
There is a need for a salt- and pressure-tolerant SAP network having high water absorbency.